netdev - Re: iproute2-2.6.20-070313 bug ?

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Message-ID: <20070322101224.3e6bb899@freekitty>
Date:	Thu, 22 Mar 2007 10:12:24 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Denys <denys@...p.net.lb>, netdev@...r.kernel.org
Subject: Re: iproute2-2.6.20-070313 bug ?

On Thu, 22 Mar 2007 14:09:06 +0100
Patrick McHardy <kaber@...sh.net> wrote:

> Denys wrote:
> > /sbin/tc2 qdisc del dev ppp0 root 
> > /sbin/tc2 qdisc add dev ppp0 root handle 1: prio 
> > /sbin/tc2 qdisc add dev ppp0 parent 1:1 handle 2: tbf buffer 1024kb latency 
> > 500ms rate 128kbit peakrate 256kbit minburst 16384 
> > /sbin/tc2 filter add dev ppp0 parent 1:0 protocol ip prio 10 u32 match ip dst 
> > 0.0.0.0/0 flowid 2:1 
> 
> 
> That is an incredible huge buffer value.
> 
> > qdisc tbf 2: parent 1:1 rate 128000bit burst 4294932937b peakrate
> 256000bit minburst 16Kb lat 4.2s
> 
> And it causes an overflow.
> 
> The limit for the TBF burst value with nanosecond resolution is
> ~ 4 * rate (10^9 * burst / rate < 2^32 needs to hold), resoluting
> in a worst-case latency of 4 seconds. I think this limit is in the
> reasonable range. Your configuration results in a worst-case
> queuing delay of 64s, and I doubt that you really want that.
> 
> Obviously its not good to break existing configurations, but I
> would argue that this configuration is broken.
> 


tc should check for overflows and doesn't. Do you want to make a patch
for the obvious cases?

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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